Classification of alcohols, phenols and ethers
NCERT Class 12 Chemistry Chapter 7 Section 7.1
Quick answer
Alcohols and phenols are classified as mono-, di-, tri- or polyhydric by the number of –OH groups. Monohydric alcohols with –OH on an sp carbon are primary, secondary or tertiary by the number of carbons on that carbon, and may be allylic (next to a C=C) or benzylic (next to an aromatic ring); with –OH on an sp carbon they are vinylic alcohols, or phenols when the carbon is in a benzene ring. Ethers are simple or symmetrical when both groups on the oxygen are the same, as in CHOCH, and mixed or unsymmetrical when they differ.
With your textbook
NCERT Chemistry Part II (Class 12), Chapter 7 “Alcohols, Phenols and Ethers”, Section 7.1 “Classification”, pages 194–195.
Open the official NCERT chapter (PDF, 4.6 MB, ncert.nic.in) (opens in a new tab)
Checked against the 2026-27 reprint on .
How are alcohols, phenols and ethers classified?
Alcohols, phenols and ethers
An alcohol has one or more –OH groups attached to carbon atoms of an aliphatic system, as in CH3OH; a phenol has –OH attached to a carbon of an aromatic ring, as in C6H5OH; and an ether has an alkoxy or aryloxy group, R–O– or Ar–O–, in place of a hydrogen of a hydrocarbon, as in CH3OCH3 (NCERT Unit 7, Introduction, pages 193–194). Alcohols and phenols are classified first by how many –OH groups they carry:
Alcohols and phenols may be classified as mono–, di–, tri- or polyhydric compounds depending on whether they contain one, two, three or many hydroxyl groups respectively in their structures as given below:
NCERT Chemistry Part II (Class 12), Section 7.1.1, page 194
In plain words: count the –OH groups, then look at the carbon that carries the –OH. An sp3 carbon gives an ordinary alcohol, graded 1°, 2° or 3° and possibly allylic or benzylic; an sp2 carbon gives a vinylic alcohol or, on a benzene ring, a phenol. Ethers are sorted by whether the two groups on the oxygen are the same.
Mono-, di-, tri- and polyhydric alcohols
- One –OH: monohydric, such as ethanol, C2H5OH. Two: dihydric, such as ethane-1,2-diol, HOCH2CH2OH. Three: trihydric, such as propane-1,2,3-triol (glycerol), HOCH2CH(OH)CH2OH. Many: polyhydric (NCERT page 194).
- Monohydric alcohols are classified further by the hybridisation of the carbon that carries the –OH (NCERT page 194), as below.
Primary, secondary and tertiary alcohols
In alcohols with a Csp3–OH bond the –OH is on an sp3 carbon of an alkyl group. It is a primary (1°), secondary (2°) or tertiary (3°) alcohol when that carbon is a primary, secondary or tertiary carbon (NCERT page 194):
| Class | The carbon carrying –OH | Example |
|---|---|---|
| 1° | –CH2OH, bonded to one carbon | CH3CH2OH |
| 2° | >CH–OH, bonded to two carbons | (CH3)2CHOH |
| 3° | >C–OH, bonded to three carbons | (CH3)3COH |
Methanol, CH3OH, has no other carbon on its –OH carbon, so it fits none of the three rows. It is grouped with the primary alcohols because it reacts like them: like ethanol, it is oxidised first to an aldehyde and then to an acid (NCERT page 210; reasoned).
Allylic and benzylic alcohols
- Allylic alcohols: the –OH is on an sp3 carbon next to a C=C double bond, an allylic carbon, as in CH2=CH–CH2OH (NCERT page 194).
- Benzylic alcohols: the –OH is on an sp3 carbon next to an aromatic ring, as in C6H5CH2OH (NCERT pages 194–195).
- Both may be primary, secondary or tertiary, depending on how many carbons the –OH carbon carries (NCERT page 195): C6H5CH2OH is 1°, C6H5CH(OH)CH3 is 2° and C6H5C(CH3)2OH is 3° benzylic (reasoned from NCERT’s drawings, page 195).
Compounds with a C(sp)–OH bond
- Here the –OH is bonded to a carbon of a C=C double bond, a vinylic carbon, or to a carbon of an aromatic ring (NCERT page 195). The simplest vinylic alcohol is CH2=CH–OH.
- NCERT slip (page 195): NCERT says that compounds with –OH bonded “to a vinylic carbon or to an aryl carbon” are “also known as vinylic alcohols”. The name fits only –OH on a carbon of a C=C double bond, as in CH2=CH–OH. With –OH on a carbon of an aromatic ring the compound is a phenol, the class NCERT Section 7.1.2 takes up next; NCERT’s own Unit 6 (page 161) likewise keeps vinylic halides and aryl halides apart. In an exam, call CH2=CH–OH a vinylic alcohol and C6H5OH a phenol.
Mono-, di- and trihydric phenols
Phenols are classified in the same way, by the number of –OH groups on the ring (NCERT page 195):
- Monohydric: phenol, C6H5OH, and 2-methylphenol (o-cresol), CH3C6H4OH.
- Dihydric: benzene-1,2-diol (catechol), C6H4(OH)2.
- Trihydric: benzene-1,2,4-triol, C6H3(OH)3, as NCERT draws it (name reasoned).
Ethers
NCERT classifies ethers by the two groups on the oxygen:
Ethers are classified as simple or symmetrical, if the alkyl or aryl groups attached to the oxygen atom are the same, and mixed or unsymmetrical, if the two groups are different.
NCERT Chemistry Part II (Class 12), Section 7.1.3, page 195
- Diethyl ether, C2H5OC2H5, is symmetrical; C2H5OCH3 and C2H5OC6H5 are unsymmetrical (NCERT page 195).
Solved examples
Example 1: Primary, secondary or tertiary?
Classify (i) (CH3)2C(OH)CH2CH3, (ii) CH3CH(OH)CH2CH3 and (iii) (CH3)2CHCH2OH.
Answer: Count the carbons bonded to the carbon that carries –OH. (i) Three (two CH3 and CH2CH3): 3°. (ii) Two: 2°. (iii) One (the CH carbon), even though the molecule is branched: 1° (NCERT page 194; reasoned).
Example 2: Allylic, benzylic, or neither?
Classify (i) CH2=CHCH(OH)CH3, (ii) C6H5C(CH3)2OH and (iii) C6H5CH2CH2OH.
Answer: (i) The –OH carbon is next to the C=C and bonded to two carbons: a 2° allylic alcohol. (ii) The –OH carbon is next to the ring and bonded to three carbons: a 3° benzylic alcohol. (iii) The –OH carbon is one carbon away from the ring, not next to it: an ordinary 1° alcohol, not benzylic (NCERT pages 194–195; reasoned).
Example 3: Symmetrical or unsymmetrical ethers
Which of CH3OCH3, CH3OC6H5 and C6H5OC6H5 are symmetrical?
Answer: CH3OCH3 and C6H5OC6H5, with the same group on each side of the oxygen, are symmetrical; CH3OC6H5 (methyl and phenyl) is unsymmetrical (NCERT page 195).
Common mistakes to avoid
Common mistake
Classifying by branching instead of by the –OH carbon
2-Methylpropan-1-ol, (CH3)2CHCH2OH, is branched, but its –OH carbon carries only one carbon: it is 1°.
Common mistake
Calling C<sub>6</sub>H<sub>5</sub>CH<sub>2</sub>CH<sub>2</sub>OH benzylic
A benzylic –OH sits on the carbon next to the ring. In 2-phenylethanol it is one carbon further away, so it is an ordinary primary alcohol.
Common mistake
Calling phenol a benzylic alcohol
In phenol the –OH is on a ring carbon, which is sp2. In a benzylic alcohol it is on the sp3 carbon next to the ring, as in C6H5CH2OH.
Quick revision summary
- By number of –OH groups: mono-, di-, tri- or polyhydric alcohols and phenols.
- C(sp)–OH: 1°, 2° or 3° by the carbons on the –OH carbon; allylic (next to C=C) and benzylic (next to an aromatic ring) alcohols can each be 1°, 2° or 3°.
- C(sp)–OH: vinylic alcohols (CH=CH–OH) and phenols (–OH on the ring).
- Ethers: simple or symmetrical (same groups, CHOCH) or mixed or unsymmetrical (different groups, CHOCH).
Formula sheet
- 1° alcohol: –CHOH; 2°: >CH–OH; 3°: >C–OH
- Allylic: CH=CH–CHOH; benzylic: CHCHOH
- Vinylic: CH=CH–OH; phenol: CHOH
- Symmetrical ether: CHOCH; unsymmetrical: CHOCH
Questions students ask
What is the difference between alcohols, phenols and ethers?
Alcohols, phenols and ethers differ in what their oxygen is bonded to: an alcohol has –OH on a carbon of an aliphatic system (CH3CH2OH), a phenol has –OH directly on an aromatic ring carbon (C6H5OH), and an ether has its oxygen between two carbon groups, R–O–R′, with no O–H (NCERT pages 193–194). So alcohols and phenols hydrogen bond to one another and boil well above ethers of similar molecular mass, and phenols are far stronger acids, dissolving in aqueous NaOH (NCERT pages 203–206). Only alcohols are graded 1°, 2° or 3°.
Why are vinylic alcohols unstable?
Simple vinylic alcohols are unstable because they turn into the isomeric carbonyl compound: CH2=CH–OH, formed when water adds to ethyne, becomes ethanal, CH3CHO, by a hydrogen shift from O to C (NCERT Class 11 Chemistry, Unit 9, page 317; alkynes). Beyond the NCERT text, ethanal is the more stable by about 43 kJ mol, and this keto–enol tautomerism is fast with any trace of acid, base or water (Wikipedia, “Vinyl alcohol”). Phenol, though its –OH is also on an sp2 carbon, stays put: its keto form would lose the ring’s aromatic stability, so only about one molecule in 10 is keto (Wikipedia, “Phenol”).
Is phenol an alcohol, and is it a secondary alcohol?
Phenol is not an alcohol: NCERT defines alcohols by –OH on a carbon of an aliphatic system and phenols by –OH on a carbon of an aromatic ring, and classifies phenols separately (NCERT pages 193 and 195), though page 195 loosely files aryl –OH compounds under “vinylic alcohols”, an NCERT slip. The 1°, 2° and 3° labels apply only to –OH on an sp3 carbon (NCERT page 194), so they do not fit phenol, even though its –OH carbon is bonded to two other carbons. Phenol also behaves unlike a secondary alcohol: chromic acid gives benzoquinone, a conjugated diketone, not a simple ketone, its C–O bond does not break with HCl, and it dissolves in aqueous NaOH (NCERT pages 205, 208 and 213).
What are examples of primary, secondary and tertiary allylic alcohols?
Primary, secondary and tertiary allylic alcohols include CH3CH=CHCH2OH (but-2-en-1-ol, 1°), CH2=CHCH(OH)CH2CH3 (pent-1-en-3-ol, 2°) and CH2=CHC(CH3)2OH (2-methylbut-3-en-2-ol, 3°) (reasoned). Allylic alcohols are graded like other alcohols, by counting the carbons bonded to the sp3 carbon that carries the –OH, the C=C carbon next to it included (NCERT pages 194–195).
Which chapter is alcohols, phenols and ethers, and is classification in the JEE Main 2027 and NEET (UG) 2026 syllabuses?
Alcohols, phenols and ethers is Unit 7 of NCERT Class 12 Chemistry Part II, the book’s second unit in the 2026–27 reprint (pages 193–226), and part of Unit 17, Organic Compounds Containing Oxygen, of both JEE Main 2027 and NEET (UG) 2026. Unit 17 lists the “Identification of primary, secondary and tertiary alcohols” (JEE Main 2027 wording), which presumes the 1°, 2° and 3° classes, and Unit 14 of both covers the “classification of organic compounds based on functional groups” (JEE Main 2027 wording). Neither names the polyhydric, allylic, benzylic or vinylic classes or symmetrical ethers, which come from NCERT Section 7.1 (pages 194–195).
Practice questions
Try each question first, then open the answer to check your working.
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Q1. Classify (CH3)3CCH2OH and (C2H5)3COH as 1°, 2° or 3°.
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(CH3)3CCH2OH: the –OH carbon carries one carbon, so 1°, even though that neighbour is bonded to four carbons. (C2H5)3COH: it carries three, so 3° (NCERT page 194; the first compound is NCERT Intext Question 7.1 (i), page 195; reasoned).
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Q2. How many –OH groups do HOCH2CH2CH2OH and HOCH2CH(OH)CH(OH)CH2OH carry, and is each mono-, di-, tri- or polyhydric?
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HOCH2CH2CH2OH, propane-1,3-diol, has two: dihydric. HOCH2CH(OH)CH(OH)CH2OH, butane-1,2,3,4-tetrol, has four: polyhydric (NCERT page 194; reasoned).
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Q3. Classify (CH3)2CHOCH(CH3)2 and CH3CH2CH2OCH(CH3)2 as symmetrical or unsymmetrical ethers.
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(CH3)2CHOCH(CH3)2 has an isopropyl group on each side of the oxygen: symmetrical. CH3CH2CH2OCH(CH3)2 has propyl on one side and isopropyl on the other: unsymmetrical, although both groups are C3H7 (NCERT page 195; reasoned).